The Miravalles geothermal power station in Costa Rica is described before and after the addition of Unit 5, a bottoming binary power plant. The early planning and design alternatives for the new unit, the bids, the selection criteria followed, and details of the final design, including the unique asymmetrical piping layout, are discussed. The performance of Unit 5 is assessed both on a cycle and an overall plant basis. Thermal and utilization efficiencies are reported. The pentane power cycle is shown to scale in pressure-enthalpy and temperature-entropy diagrams. The analysis of data on 2 years of operation indicates that, during its acceptance tests, Unit 5 showed a net overall plant thermal efficiency in excess of 16%. Much of the operating data are burdened with such scatter and uncertainty as to render conclusions difficult to draw. However, recent, more reliable data have revealed a drop-off in efficiency that was likely caused by changes in brine supply flow rate relative to the design value. The average capacity factor over the plant's first 26 months was 94% and at least one of the two modules was in operation 96% of the time. Unit 5 raised the overall Miravalles utilization efficiency by about 13% without the need for additional wells. (c) 2006 Published by Elsevier Ltd on behalf of CNR.